Even though sophisticated theories have been developed in population genetics, these are not sufficient to deal with many problems in natural and human populations. I propose the following five projects. (1) Population dynamics of deleterious genes under social selection. Existing data in man show that there exists complex interactions between an individual's fitness and that of his relatives. Analyses of social selection will add a new dimension not only in the study of mutational damage in man but also in the study of the effect of social structre on the change in gene frequency in other organisms. (2) Mechanism of maintenance of deleterious genes in a geographically structured population. Genetic correlation of deleterious genes between two populations is not well understood. When the genetic correlation is known, it can be used in the computation of the rate of allelism of lethal chromosomes for Drosophila. (3) Population dynamics of deleterious genes in an age-structured population. Existing genetic theories with overlapping generations are generally constructed under the assumption of haploidy or infinitely large population. To apply theories to data, it is necessary to incorporate more realistic assumptions. (4) Mechanism of maintenance of variation in the frequency of genetic diseases in human isolates. This problem is important to understand not only for detecting the major factor which contributes to the high frequency of disease but also for understanding the significance of polymorphism in evolution. (5) Mechanism of sex chromosome differentiation. The mechanism of sex chromosome differentiation is still speculative. Rigorous theoretical analyses of existing hypotheses will provide important new approaches to this problem.